Debris flows originating from colluvium deposits in hollow regions during a heavy storm process in Taining, southeastern China

Abstract

On 8 May 2016, eight debris flows occurred in Taining County, Fujian Province, China. Thirty-six people died as a result of the largest catastrophic debris flow (DF3), which occurred in the Luankeng gully and has attracted the attention from scholars all over the world. A field investigation was conducted, and site measurements were obtained to identify the processes of debris flow formation, movement, and deposition. The precipitation seepage was analyzed by numerical simulation with the Geo-Studio software. The mechanism that transforms the colluvium deposits in hollow regions (CDHs) into debris flows was studied by a stability analysis of the CDH in the initiation zone. The results showed the following: (1) the CDH instability was triggered by the combined effects of rainfall and flow concentration at the end of the hollow, and the CDH slide occurred along the interface between the bedrock and hollow, which transformed into a debris flow; (2) the debris flow discharge was amplified 6.5 times due to blockage by boulders distributed in the gully; and (3) the confluent area of the hollow region and the CDH slope gradient and thickness impart significant impact on the CDH stability. The CDH will experience failure when the slope gradient and thickness exceed the critical values of 30.5° and 1.25 m, respectively. The confluent area of the hollow may supply as much as four times the normal amount of water, which may trigger a debris flow during heavy rainfall.